Method and apparatus for producing hydrogen using solar energy

ABSTRACT

Apparatus suitable for use as a solar energy collector, the apparatus comprising a closed loop through which, in use, an aqueous solution can be circulated and be impinged upon by incident sunlight; means for circulating the aqueous solution around the closed loop; an inlet for introducing reactants into the closed loop; at least one unit in which the aqueous solution can be subjected to a chemical reaction, the said unit being interposed in the closed loop in a manner such that the said solution can flow through the unit; and an outlet for withdrawing products from the closed loop. The use of such apparatus to perform the photochemical oxidation of water to give oxygen and a reduced electron acceptor which acts as a store of energy is also described.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and a method for thecollection of solar energy and for the conversion of the collectedenergy into chemical fuel.

2. Description of Prior Art

An economic system for the collection of solar energy requires acollector with a large surface area. Assuming an efficiency ofcollection of about 10%, the collector must cost less than 10 poundssterling per square meter at present day (1977) prices if it is tocollect energy (other than low grade heat energy) at a price competitivewith that provided by a modern power station. Furthermore, the energyshould preferably be produced in storable form.

No such collecting system is known today which costs less than 10 poundssterling per square meter. Photovoltaic cells cost at least ten timesthis price and do not enable the energy to be stored. Block platesthrough which water is circulated are more economic but provide only lowgrade heat and only give short term storage.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to provide an economically viableapparatus and method for the collection of solar energy.

According to one aspect of the present invention there is provided anapparatus suitable for use as solar energy collector, the apparatuscomprising a closed loop through which, in use, an aqueous solution canbe circulated and be impinged upon by incident sunlight; means forcirculating the aqueous solution around the closed loop; an inlet forintroducing reactants into the closed loop; at least one unit in whichthe aqueous solution can be subjected to a chemical reaction, the saidunit being interposed in the closed loop in a manner such that the saidsolution can flow through the unit; and an outlet for withdrawingproducts from the closed loop.

The closed loop can be formed from any suitable material provided thatthe surface of the closed loop which faces the impinging sunlight istransparent to the sunlight. Naturally the larger the surface area ofthe closed loop upon which sunlight is incident, the greater will be theamount of solar energy collected by the system. Preferably the closedloop is made of a flexible material such as flexible plastics sheet sothat it can be easily transported and assembled.

One convenient form of the closed loop is a plurality of prefabricatedcollection mats, preferably rectangular, each having an inlet port andan outlet port by means of which the mats can be connected one toanother in sequence. Any number of the mats can be connected together toproduce a total surface area of any required size. In this way manyacres of desert, wasteland or roof surface can be covered convenientlyand any one unit can be replaced easily should it be damaged.

The means for circulating the solution is preferably a pump which mayconveniently be operated by solar energy, for example on the fluidyneprinciple, so that no external power is needed to operate the system.

The time for the solution to complete one cycle around the closed loopis typically half an hour, but depending on the concentration of thereactants in the solution and the intensity of the incident sunlight, itcould be an order of magnitude smaller or greater than this. With atypical apparatus having a cyclic pathway 100 meters in overall length,a cycle time of half an hour would correspond to a flow rate of about 3meters per minute.

According to a further aspect of the present invention there is provideda method for the collection of solar energy by photo-oxidation of waterand for the conversion of such energy into a chemical fuel, which methodcomprises forming an aqueous solution of an absorbing substance D, areducing substance R and an oxidising substance Ox, the substances D, Rand Ox being chosen so that upon irradiation by sunlight dissociationand photo-oxidation of water takes place as a result of D absorbinglight and the resultant excited state of D reacting with R, Ox and waterto oxidise R to R_(ox) and reduce Ox to Ox_(R) ; circulating the aqueoussolution through the closed loop of an apparatus according to theinvention while irradiating the solution with incident sunlight therebyto cause said photo-oxidation of water; at a first station in the closedloop, regenerating Ox from Ox_(R) with liberation of hydrogen as thechemical fuel; optionally, at a second station in the closed loop,regenerating R from R_(Ox) with concomitant release of oxygen; andintroducing water into the closed loop to replace dissociated water.

The substances D, R and Ox are chosen so that in the overall reactionwhich takes place, D, R and Ox are not consumed and water is convertedinto hydrogen and oxygen.

The absorbing substance D which is a dye or a pigment may be permanentlybound either to R or to Ox so as to improve the efficiency of thereaction. Furthermore one of the reactants, preferably that containingthe substance D, may be bound to the surface of a solid carrier so thatit is fixed in position and does not circulate with the aqueoussolution.

In a preferred embodiment of the present method, D and R are boundtogether chemically. Preferably, the method of the present invention isperformed by making use of the photochemical process for the catalyticphoto-oxidation of water described in U.S. Ser. No. 895,555 filed Apr.12, 1978. In this process D and R are bound together chemically and takethe form of an organic complex of manganese while Ox takes the form ofan electron acceptor such as quinone or certain specified heterocycliccompounds. More specific details of this preferred photochemical processare given below.

The chemical reactants such as D, R and Ox need not always be insolution. In an alternative arrangement there is included within theclosed loop a solid carrier to the surface of which is fixed one or morechemical reactants. Preferably the solid carrier is constituted bystrips of flexible material impregnated with the chemical reactant andanchored at one end to the closed loop, or by an inert material such asa zeolite impregnated with the chemical reactant.

It is a particular feature of the method of the present invention thatit enables the oxidised material R_(Ox) and the reduced material Ox_(R)to be easily treated to regenerate R and Ox respectively. Furthermore,the present method enables the fuel to be released at a convenientlocation. This is especially an advantage when the fuel is hydrogen gas.In the arrangement of the present invention, the reduced material isproduced in the aqueous solution during solar irradiation of thesolution as the solution travels along the closed loop, but hydrogen isliberated from the reduced material at one or more chosen locations onlyin the closed loop. This is achieved by passing the irradiated aqueoussolution through one or more units interposed in the loop, each of whichis capable of converting Ox_(R) into hydrogen gas or hydrogen in someconvenient bound form with consequent regeneration of Ox. Hydrogen gasmay pass immediately into a gas supply line, whilst a bound form ofhydrogen may be transported in liquid or solid form for use elsewhere asa fuel.

In some systems, as the solution passes along the loop, oxygen isautomatically released from the oxidised material and allowed to escape.In these cases the oxygen release unit or units are not necessary. Inother systems, if the oxygen is required as a product or if itinterferes with the rest of the chemical cycle, it may be released atone or more locations only in the loop by one or more catalytic oxygenrelease units in which the reducing substance is regenerated and oxygenis liberated.

BRIEF DESCRIPTION OF THE DRAWINGS

The apparatus and method of the present invention will now be described,by way of example only, with reference to the following drawings, inwhich:

FIG. 1 is a diagrammatic plan of one embodiment of the apparatus of thepresent invention;

FIG. 2 is a section along the line II--II of the apparatus shown in FIG.1; and

FIG. 3 is a section of a mat similar to that shown in FIG. 2, but ofdifferent construction.

In the drawings, like reference numerals refer to like parts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1 and 2 there is shown a closed loop, indicatedgenerally at 2, comprising two prefabricated collection mats 4, 6connected in sequence. Although only two mats are shown, any desirednumber of mats may be connected together in sequence provided that thepump 8 has sufficient power to circulate a liquid solution around theclosed loop formed by the interconnected mats. The mats shown in thedrawings are made of an upper sheet 10 of flexible plastic materialwelded to a base 12 at each side 14, 16 and at each end 18, 20. Each mathas an inlet port 22 at one end and an outlet port 24 at the oppositeend, for respective entry and exit of the solution which, in operation,passes through each mat. Suitable plastics materials for the upper sheet10 are cellophane, polyethylene, poly-propylene, poly-phenyleneoxide,poly-amides, polyesters, or glass reinforced plastics, although anyinexpensive transparent material which is resistant to sunlight would besuitable. The base 12 is also flexible and impermeable to water. Asuitable material for the base 12 is rubber re-inforced bitumen of thetype presently used in the desert to reduce the shifting of sand. Theupper 10 is also welded to the base along spaced apart longitudinallines 28 parallel to the sides of the sheets in order to form a seriesof longitudinal channels 26 which guide the flow of the solution throughthe mat. The height of each channel 26, when full of solution, istypically from 1 to 10 cm. When suitable materials, welding of the uppersheet 10 to the base 12 can be achieved merely by heating at therequired sealing positions.

Preferably the surface of the base 12 which faces the incident sunlightis covered with white paint or another suitable white material toreflect the incident rays of sunlight back through the aqueous solution.

An alternative construction of mat is shown in FIG. 3 in which an uppersheet 10 of the type described in FIG. 2 is heat sealed to a base 12formed from a sheet of the same material as upper sheet 10. With suchmats, it is envisaged that some form of transparent cover sheet 29 mightbe necessary to prevent accumulation of dust, sand, grit and otherundesirable extraneous matter in the resultant grooves 30 between eachchannel 26 since the presence of such extraneous matter will reduce theefficiency of the system.

The flexibility of the collection mats makes them easy to transport andassemble.

The closed loop is completed by pipes 32, 34, 36 linking the outlet port24 of the second mat 6 to the inlet port 22 of the first mat 4 via ahydrogen release unit 38, an optional oxygen release unit 40 and thepump 8 for circulating the solution around the closed loop. The pump ispreferably positioned so that it creates a positive pressure in the matsrather than a negative pressure. With such an arrangement, if a matdevelops a leak, the solution slowly leaks out until the leak is stoppedrather than air being drawn into the system.

In a preferred method of operation of the present apparatus, thecatalytic photo-oxidation of water is achieved by circulating an aqueoussolution containing substances D, R and Ox through mats 2 and 4. Dabsorbs sunlight and the excited D reacts with R, Ox and water so that Ris oxidised to R_(Ox) and Ox is reduced to Ox_(R). When the solutionreaches unit 38, hydrogen is released from Ox_(R) and Ox is regenerated.The released hydrogen is conveyed to a suitable collection point viaconduit 42. Similarly, in some processes when the solution reaches unit40, oxygen is released and R is regenerated. The liberated oxygen passesalong conduit 44 for collection or release.

The solution is then recycled to collection mat 2 via pump 8, anappropriate quantity of water having been added to the closed loop viainlet 46 from a source 48 to make up for the water which has dissociatedduring the preceding circulation of the solution around the closed loop.

A catalytic process for the photo-oxidation of water preferred for usein the present method is that described in U.S. Ser. No. 895,555 filedApr. 12, 1978.

This application describes and claims a process for the catalyticphoto-oxidation of water by visible light which process comprisesirradiating with light in the visible region of the spectrum a solutioncomprising:

(i) water,

(ii) as photo catalyst, a starting manganese complex which is amanganese (II) or manganese (III) complex of formula MnL wherein L is aphthalocyanine ligand, a 5, 10, 15, 20-tetraphenylporphyrin (TPP) ligandor a 5, 10, 15, 20-tetrapyridyl porphyrin (TPyP) ligand, each of saidligands optionally being substituted by further substituents, the saidstarting manganese complex being capable of undergoing a transition intoan excited state on irradiation with visible light with subsequentconversion to the corresponding manganese (IV) complex, and

(iii) a compound which is an electron acceptor and which is asufficiently powerful oxidising agent to abstract electrons from theexcited manganese complex, the pH of the reaction mixture being chosenso that upon irradiation, the starting manganese complex will beoxidised to the corresponding manganese (IV) complex and the electronacceptor will be reduced to form a reduced electron acceptor and sothat, subsequently, the manganese (IV) complex will be reduced back tothe corresponding manganese (II) complex with concomitant conversion ofwater into oxygen.

Thus this preferred photochemical process is carried out underconditions such that Mn (II) and Mn (III) can be photo-oxidsed to Mn(IV) which is a powerful oxidant and which can react, photochemically orthermally, to liberate O₂ from water and itself return to the startingmanganese complex. The overall reaction is the oxidation of water tooxygen and the simultaneous reduction of an electron acceptor to form acompound which acts as a store of energy and which may subsequently beused as, or converted to, a chemical fuel.

Both the manganese complex and the electron acceptor must either bewater soluble compounds or must be capable of being solubilised into amicellar solution by the presence in the compound of suitable ligandssuch as long aliphatic chains which are hydrophobic or by the use ofmicelle-forming surfactants such as sodium lauryl sulphate orcetyltrimethyl ammonium bromide. In this way the required aqueoussolutions of reactants can be formed. When the manganese complex ispresent in the form of micelles, then it is usual to use a water-solubleelectron acceptor. Similarly if the electron acceptor is present in theform of micelles, the manganese complex used is usually one which iswater-soluble.

The starting manganese complex used in this preferred photochemicalprocess will absorb light in the visible region of the spectrum, that isto say will absorb energy from sunlight, and will undergo aphoto-oxidation reaction to form the corresponding manganese (IV)complex, which at an appropriate pH will react, thermally orphotochemically, with water to give oxygen.

The phthalocyanine and porphyrin ligands preferably contain at leasttwo, and generally up to eight, water-solubilising groups. By a"water-solubilising group" is meant a group such as a carboxy group orsulpho group which increases the hydrophilic character of a compound towhich it is attached. There are usually four water-solubilising groupspresent in each ligand, one on each of the four benzene rings of eachligand. Particularly preferred starting manganese complexes are aphthalocyanine complex of formula: ##STR1## wherein R is a --CO₂ H or a--SO₃ H group; or a tetraphenylporphyrin complex of formula: ##STR2##wherein R is also a --CO₂ H or --SO₃ H group, and R" is a C₁ to C₄ alkylgroup. It has also been found that when the --CO₂ H groups mentionedabove are replaced by groups such as --CO₂ R', or when the --SO₃ Hgroups mentioned above are replaced by --SO₂ NHR', wherein R' is a C₁₆to C₂₂ alkyl group, especially a C₁₈ alkyl group, the resultant complex,although not water soluble, will form micelles in aqueous solution andenable the photochemical process to take place.

Examples of preferred tetrapyridylporphyrin complexes are the watersoluble complex of formula: ##STR3## wherein each R"' group is a C₁₋₄alkyl group, preferably a methyl group, and A⁻⁻ is a suitable anion suchas iodide, and the corresponding free-bases which are also watersoluble. The corresponding N, N', N", N"'-tetra C₁₆ to C₂₂ -alkylderivatives are also suitable; these compounds are not water soluble,but form a dispersion of micelles in water.

Examples of suitable electron acceptors are quinones such asbenzo-1,4-quinone and anthra-9,10-quinone, each optionally substitutedwith one or more atoms or groups chosen from alkyl, chloro, sulpho,phenylsulphonyl and cyano atoms or groups; a 1,4-naphthaquinone; certainheterocyclic compounds such as pyrazine and mono-, di-, tri- ortetramethylpyrazine, quinoxaline, phenazine, N-methylphenazine sulphate,NADH, methyl viologen and benzyl viologen, and dyes such as thionine andmethylene blue.

Particularly preferred benzo-1,4-quinones are those of general formula(I) ##STR4## wherein each of R¹, R², R³ and R⁴ independently representsa hydrogen or a chlorine atom, or a methyl, sulpho, phenylsulphonyl orcyano group. In addition, when the quinone is to be incorporated inmicelles, each of R¹, R², R³ and R⁴ may independently represent a higheralkyl group, particularly a C₁₆ to C₂₂ alkyl group.

Particularly preferred anthra-9,10-quinones are those of general formula(II): ##STR5## wherein each of R¹ to R⁸ independently represent ahydrogen or chlorine atom or a methyl, sulpho, phenylsulphonyl, cyano orcarboxy group.

More preferred are anthra-9,10-quinones of general formula (II), whereineach of R¹ and R² independently represents a hydrogen atom or a sulphoor carboxy group, and each of R⁵ and R⁶ independently represents ahydrogen atom, or a sulpho or carboxy group.

With an homogeneous aqueous solution, the electron acceptor is chosen tobe soluble and stable in water at the pH used for the process. If the Mncomplex is water-insoluble and is solubilised by use of a micelleformingmaterial then, again, the electron acceptor must be water-soluble andstable at the pH used. Electron acceptors that are insoluble in water,such as 2,3,5,6-tetrachloro-1,4-benzoquinone, are solubilised in amicelleforming material and are used in conjunction with a water solubleMn complex. In such cases, the maganese complex or the electron acceptoris formed into a micellar solution.

In most cases, the concentration of electron acceptor is controlled sothat the molar ration of electron acceptor

to Mn complex is of the order of 100:1. However, in general, the molarratio of electron acceptor to manganese complex can be as low as 10:1.Moreover, any desired excess of electron acceptor can be used. Theseconditions give a large excess of electron acceptor and increase theyield of products.

In the preferred photochemical process both oxidation and reductionsteps are dependent upon pH, because the pH affects the redox potentialsof the manganese complexes. In general, oxidation requires higher pHconditions whilst reduction proceeds more readily in acidic solution.The pH of the reaction solution is chosen so that an efficient cyclicreaction system and optimum yields of photoproducts are obtained.

In general, the most suitable pH range is found to be 4 to 11, morepreferably 7 to 9.

In an alternative arrangement the absorbing substance D, or the (D+R)combination, instead of being dissolved in the aqueous solution, isfixed to the surface of a solid carrier suspended in that solution. As afirst example, manganese porphyrin is incorporated as a dye onto thesurface of a thin polymer sheet. Strips 50 of this sheet (only one setof such strips being shown in FIG. 1), anchored at one end, aresuspended in the flowing solution of the oxidising substance Ox. As asecond example, the manganese porphyrin is fixed on an inorganicsubstance, such as a zeolite. This arrangement has the advantage thatonly the substance Ox and its reduced form are able to flow into thehydrogen generating unit 38 thus preventing interference by D or (D+R)with the catalytic or electrolytic operation of that unit.

Release of hydrogen in the hydrogen generating unit 38 can be achievedby generating hydrogen from the reduced electron acceptor by severalmethods, most of which are known. For example:

(i) Reaction of the reduced electron acceptor with an enzyme catalystsuch as hydrogenase or a synthetic iron sulphur protein or an inorganiccatalyst, e.g. a metal such as platinum. This method is suitable whenthe electron acceptor is for example methyl viologen (MeV²⁺) or benzylviologen, thionine or methylene blue. The reaction can be representedas: ##EQU1##

(ii) Electrolytic oxidation of the reduced electron acceptor. Thismethod is suitable when the electron acceptor is for example a quinoneor one of the heterocyclic compounds mentioned above. The oxidation canbe represented as:

    QH.sub.2  .sup.2e  Q+H.sub.2

It should be noted that although energy is expended in this process, theamount required is less than for direct electrolysis of water.

When the processes described in U.S. Ser. No. 895,555 filed Apr. 12,1978 are used for the photo-oxidation reaction, oxygen is releasedthrough-out the flow system and unit 40 is therefore not required.However, when a unit 40 is used to release oxygen this can suitably beachieved by using an acid ion exchange resin in unit 40. It is known forexample that oxidised manganese (IV) phthalocyanine sulphonate is stablein alkaline solution but liberates oxygen and returns to the manganese(II) form in acid. This effect can be used to liberate oxygen only atthe specific point in the cycle where the acid catalyst is. This is animportant advantage since it enables the photochemical part of the cycleto be performed under anaerobic conditions if so desired.

Thus it will be seen that important advantages of the present inventionare that no chemical substances other than water are consumed, that itis a continuous cyclic flow system requiring no attention other thanmaintenance, and that it enables the oxidised and reduced products whichare synthesised over a large surface area during the circulation aroundthe closed loop to be separated from each other and be collected at aconvenient location. Moreover the light collecting mat, which must ofnecessity be very large to collect solar energy over large areas, can beof low cost, for example 1 pound sterling per square meter.

What is claimed is:
 1. A unidirectional continuous flow solar energycollector comprising a closed loop through which, in use, an aqueoussolution can be circulated continuously in one direction only and beimpinged upon by incident sunlight while circulating through the saidloop; means for circulating the aqueous solution continuously in onedirection through the closed loop; an inlet for introducing reactantsinto the closed loop; at least one station comprising a means for theliberation of hydrogen derived from the water of the aqeuous solution,the hydrogen being in gaseous or combined form, said station beinginterposed in the closed loop in a manner such that the aqueous solutioncan flow through the station; and an outlet for withdrawing productsfrom the closed loop.
 2. Apparatus according to claim 1, wherein theclosed loop is made of a flexible material.
 3. Apparatus according toclaim 2, wherein the material is flexible plastics sheeting. 4.Apparatus according to claim 1, wherein the closed loop includes aplurality of prefabricated collection mats, each having an inlet portand an outlet port by means of which the mats can be connected one toanother in sequence.
 5. Apparatus according to claim 4, wherein each matis formed from an upper transparent sheet attached at its periphery influid tight manner to a base sheet.
 6. Apparatus according to claim 5,wherein the upper sheet is also attached to the base sheet along spacedapart longitudinal lines disposed inwardly from the periphery of thesheets thereby to form a series of longitudinal channels which in useguide the flow of the solution through the mat.
 7. Apparatus accordingto claim 5, wherein the surface of the base sheet which in use faces theincident sunlight is a reflective surface.
 8. Apparatus according toclaim 5, wherein the upper sheet and the base sheet of the mat are bothof the same material.
 9. Apparatus according to claim 5, wherein thebase sheet of the mat is rubber reinforced bitumen.
 10. Apparatusaccording to claim 4, which in addition includes a transparent coversheet placed over each collection mat.
 11. Apparatus according to claim4, wherein the means for circulating the solution is positioned so thatit creates a positive pressure in the mats.
 12. Apparatus according toclaim 1, further including within the closed loop a solid carrier to thesurface of which is fixed one or more chemical reactants.
 13. Apparatusaccording to claim 12, wherein the solid carrier is constituted bystrips of flexible material impregnated with the chemical reactant andanchored at one end to the closed loop.
 14. Apparatus according to claim13, wherein the flexible strips are of plastics material.
 15. Apparatusaccording to claim 12, wherein the solid carrier is constituted by aninert particulate material impregnated with the chemical reactant. 16.Apparatus according to claim 15, wherein the support is a zeolite. 17.Apparatus according to claim 1, wherein the means for circulating thesolution is a pump.
 18. Apparatus according to claim 17, wherein thepump is operated by solar energy.
 19. Apparatus according to claim 18,wherein the pump is one whose operation is based on the fluidyneprinciple.
 20. Apparatus according to claim 1, wherein said closed loopcomprises a zone where said aqueous solution is impinged upon with saidincident sunlight and a separate zone wherein hydrogen which isgenerated is liberated from the thus irradiated aqueous solution. 21.Apparatus according to claim 20, wherein said closed loop furthercomprises a zone wherein oxygen which is generated is liberated from theirradiated aqueous solution.
 22. A method for the collection of solarenergy by photo-oxidation of water and for the conversion of such energyinto a chemical fuel, which method comprises: (i) forming an aqueoussolution of an absorbing substance D, a reducing substance R and anoxidising substance Ox, the substances D R and Ox being chosen so thatupon irradiation by sunlight dissociation and photo-oxidation of watertakes place as a result of D absorbing light and the resultant excitedstate of D reacting with R, Ox and water to oxidise to R to R_(Ox) andreduce Ox to Ox_(R) ; (ii) causing the said aqueous solution tocirculate in one direction around the closed loop of a unidirectional,continuous flow solar energy collection apparatus while irradiating thesolution with incident sunlight to thereby cause said photo-oxidation ofwater; the said solar energy collection apparatus comprising a closedloop through which, in use, an aqueous solution is continuouslycirculated in one direction only and is impinged upon by incidentsunlight while circulating the aqueous solution continuously in onedirection around the closed loop, an inlet for introducing reactantsinto the closed loop, at least one first station comprising means forregenerating Ox from Ox_(R) with liberation of hydrogen as chemicalfuel, an optional at least one second station comprising means forregenerating R from R_(Ox) with concomittant release of oxygen, the saidfirst and second stations being interposed in the closed loop in amanner such that the said aqueous solution flows through the first andsecond stations, and an outlet for withdrawing products from the closedloop; (iii) at the said first station in the closed loop, regeneratingOx from Ox_(R) with liberation of hydrogen as the chemical fuel; (iv)optionally at the said second station in the closed loop, regenerating Rfrom R_(Ox) with concomitant release of oxygen; and (v) introducingwater into the closed loop to replace dissociated water.
 23. A methodaccording to claim 22, wherein the aqueous solution is circulated at arate of 3 meters per minute.
 24. A method according to claim 22, whereinthe said liberation of hydrogen is performed using an enzyme catalyst,by an electrolytic oxidation process, or by a catalysed oxidationprocess.
 25. A method according to claim 22, wherein the said release ofoxygen is performed using a catalyst which is an acid ion-exchangeresin.
 26. The process according to claim 22, wherein said aqueoussolution has a pH of 7 to 9.